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halrmt: Allow IPv4 as fallback when IPv6 is disabled. #4405

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halrmt: Allow IPv4 as fallback when IPv6 is disabled. #4405

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58 changes: 45 additions & 13 deletions src/hal/utils/halrmt.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -68,6 +68,7 @@ using namespace linuxcnc;
static std::string helloPwd = "EMC"; // Connect password
static std::string enablePwd = "EMCTOO"; // Enable password
static std::string serverName = "EMCNETSVR"; // Server name written in hello response
static bool isipv4 = false; // We try AF_INET6, but can fallback to AF_INET
static int port = 5006;
static std::string inifilename;
static volatile int quitloop = 0; // Signal to main loop to exit
Expand Down Expand Up @@ -2886,19 +2887,41 @@ static int initSocket()
int optval = 1;
int err;
int sockfd;
struct sockaddr_in6 address = {};
union {
struct sockaddr_in6 addr6;
struct sockaddr_in addr4;
} addr;

// Using AF_INET6 will also allow IPv4 to connect (v4-mapped-on-v6)
// We will try IPv6 first and hope it is available
addr.addr6 = {};
addr.addr6.sin6_family = AF_INET6;
addr.addr6.sin6_addr = IN6ADDR_ANY_INIT;
addr.addr6.sin6_port = htons(port);
socklen_t slen = sizeof(addr.addr6);

sockfd = socket(AF_INET6, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);

@grandixximo grandixximo Aug 19, 2026

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Is EAFNOSUPPORT the only failure worth falling back on? EACCES/EPERM from a
restricted netns or LSM policy reach the same point, and bind() to :: can fail
independently of socket(). Reason not to fall back on any IPv6 failure?

if (sockfd < 0) {
xperror("socket()");
return -1;
if(EAFNOSUPPORT == errno) {
// Someone disabled IPv6 on the machine. Try IPv4 instead.
sockfd = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
if (sockfd < 0) {
xperror("IPv4 socket()");
return -1;
}
isipv4 = true;
addr.addr4 = {};
addr.addr4.sin_family = AF_INET;
addr.addr4.sin_addr.s_addr = INADDR_ANY;
addr.addr4.sin_port = htons(port);
slen = sizeof(addr.addr4);
} else {
xperror("IPv6 socket()");
return -1;
}
}
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval));

@grandixximo grandixximo Aug 19, 2026

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Same as #4403: this depends on net.ipv6.bindv6only being 0. With that sysctl at
1, socket(AF_INET6, ...) succeeds so the fallback never fires and IPv4 clients
get ECONNREFUSED. Confirmed:

v6only=0  IPv4 client: connected OK
v6only=1  IPv4 client: connect FAILED (Connection refused)

Explicit setsockopt(sockfd, IPPROTO_IPV6, IPV6_V6ONLY, &zero, ...) on the IPv6
branch?

// Using AF_INET6 will also allow IPv4 to connect (v4-mapped-on-v6)
address.sin6_family = AF_INET6;
address.sin6_addr = IN6ADDR_ANY_INIT;
address.sin6_port = htons(port);
err = bind(sockfd, reinterpret_cast<struct sockaddr *>(&address), sizeof(address));
err = bind(sockfd, reinterpret_cast<struct sockaddr *>(&addr), slen);
if (err) {
close(sockfd);
xperror("bind()");
Expand Down Expand Up @@ -2985,8 +3008,11 @@ static int sockMain(int svrfd)
} else if (pfds[0].revents & POLLIN) {
// POLLIN on a listen socket means new connection available
int cfd;
struct sockaddr_in6 csa;
socklen_t csal = sizeof(csa);
union {
struct sockaddr_in6 csa6;
struct sockaddr_in csa4;
} csa;
socklen_t csal = isipv4 ? sizeof(csa.csa4) : sizeof(csa.csa6);

@grandixximo grandixximo Aug 19, 2026

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accept4() writes the real length back, so sizeof(csa) works for both families.
Would sockaddr_storage plus getnameinfo(..., NI_NUMERICHOST | NI_NUMERICSERV)
remove the union, isipv4, and the duplicated logging branch below altogether?

cfd = accept4(svrfd, reinterpret_cast<struct sockaddr *>(&csa), &csal, SOCK_CLOEXEC | SOCK_NONBLOCK);
if (cfd < 0) {
switch (errno) {
Expand Down Expand Up @@ -3028,9 +3054,15 @@ static int sockMain(int svrfd)
cr.echo = true;
cr.inifilename = inifilename; // Inherit global default
clients.push_back(cr);
char addr[INET6_ADDRSTRLEN] = {};
inet_ntop(AF_INET6, &csa.sin6_addr, addr, sizeof(addr));
info("New connection from %s:%d", addr, ntohs(csa.sin6_port));
if(isipv4) {
char addr[INET_ADDRSTRLEN] = {};
inet_ntop(AF_INET, &csa.csa4.sin_addr, addr, sizeof(addr));
info("New connection from %s:%d", addr, ntohs(csa.csa4.sin_port));
} else {
char addr[INET6_ADDRSTRLEN] = {};
inet_ntop(AF_INET6, &csa.csa6.sin6_addr, addr, sizeof(addr));
info("New connection from %s:%d", addr, ntohs(csa.csa6.sin6_port));
}
} else {
close(cfd);
}
Expand Down